The use of ultrasonic flowmeters has acquired increasing significance in the case of industrial flow measurement of liquids and gases, collectively referred to herein as flowing media. The flow measurement takes place--as in the case of magnetic inductive flowmeters--"contactlessly", that is, without disturbing parts installed in the flow, which always result in eddy formation and a high pressure loss.
In the case of ultrasonic flowmeters, with respect to the measuring process, in the first place there are the delay process and the Doppler process. In the case of the delay process, there are the direct delay difference process, the pulse recurrence frequency process, and the phase shift process (cf. H. Bernard "Ultraschall-Durchflu.beta.messung" in "Sensoren Me.beta.aufnehmer", published by Bonfig/Bartz/Wolff in the expert verlag, further the VDI/VDE GUIDELINE 2642 "Ultraschall-Durchflu.beta.messung von Flussigkeiten in voll durchstromten Rohrleitungen").
It is functionally necessary for ultrasonic flowmeters of the type under discussion, on the one hand, to have a measuring tube which as a rule represents the measuring section together with an entrance section and an exit section, and, on the other hand, at least one ultrasonic transducer called a measuring head. In this case, "ultrasonic transducer" is to be understood very generally. In the first place, on the one hand, ultrasonic transducer includes ultrasonic transmitters, therefore measuring heads for generating and for radiating ultrasonic signals, and on the other hand, ultrasonic receivers, therefore measuring heads for receiving ultrasonic signals and for converting the ultrasonic signals received into electrical signals. However, ultrasonic transducer also includes measuring heads which combine ultrasonic transmitters and ultrasonic receivers, which therefore serve both for generating and radiating ultrasonic signals, as well as for receiving ultrasonic signals and converting the ultrasonic signals received into electrical signals.
An ultrasonic transducer or measuring head of the type described last is used in ultrasonic flowmeters for flowing media, which work with only one ultrasonic transducer. Such ultrasonic flowmeters determine the velocity of the flowing medium with the help of the Doppler delay of the ultrasonic signal reflected at an inhomogeneity of the flowing medium. It is likewise conceivable, that the Doppler delay of the ultrasonic signals is determinable by two ultrasonic transducers, which are installed on opposite sides of the measuring tube without offset with respect to one another. Measurements based on the delay process are possible as well, in which two ultrasonic transducers are installed on the same side of the measuring tube arranged offset in the direction of flow, in which in this case the ultrasonic signals are reflected at the side of the measuring tube facing the ultrasonic transducers. Normally however two ultrasonic transducers are provided which are arranged on opposite sides of the measuring tube offset with respect to one another in the direction of flow. In the following, the invention is described with respect to the last explained type of ultrasonic flowmeter, without limitation to this type.
There exist on the one hand ultrasonic flowmeters, whose ultrasonic transducers do not have contact with the flowing medium, therefore being attached to the measuring tube from outside, the so called "clamp-on-arrangement". On the other hand there exist ultrasonic flowmeters, whose ultrasonic transducers have contact with the flowing medium. The invention relates only to such ultrasonic flowmeters whose ultrasonic transducers have contact with the flowing medium.
At the outset, it was said, in the case of the ultrasonic flowmeters under consideration, the ultrasonic transducers are installed in transducer pockets of the measuring tube. A transducer pocket here means a recess or hollow lying outside the flow cross-section of the measuring tube, however realized, in which an ultrasonic transducer is installed so that it does not project into the flow cross-section of the measuring tube, and therefore actually does not influence the flow. Since the ultrasonic transducers are arranged offset with respect to one another in the direction of flow, but also aligned with one another, as a rule the longitudinal axis of the transducer pockets runs at an acute angle or at an obtuse angle to the direction of flow of the flowing medium or to the longitudinal axis of the measuring tube (cf. Fig. 6.1.1 on page 532 of the reference "Sensoren Me.beta.aufnehmer", loc. cit., Fig. 8 on page 18 of the VDI/VDE-GUIDELINE 2642 "Ultraschall-Durchflu.beta.messung von Flussigkeiten in voll durchstromten Rohrleitungen", and Fig. 2-2 on page 212 of the reference "Ultrasonic Measurement for Process Control" by Lawrence C. Lynnworth, ACADEMIC PRESS, INC., published by Harcourt Brace Jovanovich).
In the case of ultrasonic flowmeters, the transducer pockets influence the flow of the medium flowing in the measuring tube; in particular, eddies are generated, having the frequency ##EQU1##
where
S=Strouhal number, PA1 V=speed of the flowing medium, PA1 D=size of the transducer pocket.
The publication "Boundary-Layer Theory" by Dr. Hermann Schlichting, McGRAW-HILL BOOK COMPANY, is to be consulted here.
The following examination shows the effect of the eddies generated by the transducer pockets:
The Strouhal number is around 0.2 and changes little when the Reynolds number is between 2.times.10.sup.2 and 6.times.10.sup.5 (cf. Fig. 2.9 on page 32 of the reference "Boundary-Layer Theory", loc. cit.). Normally, piezo-electric ultrasonic transducers are used which have a diameter of 10 to 20 mm, that is, the size of the hollow lies between 15 and 40 mm. In the case of speeds of the flowing medium between 0.5 and 10 m/s, the frequency of the eddies generated by the transducer pockets is between 2.5 and 133 Hz. Now if measurement is to be performed with an accuracy of 0.1%, then the time constant is between around 3.8 s and around 200 s. The dynamics of the ultrasonic flowmeters under consideration therefore is poor.
In order to solve the problem presented previously in detail, which results from the eddies generated by the transducer pockets, it has already been proposed that the transducer pockets be filled with plastic (cf. FIG. 4-9 on page 257 of the reference "Ultrasonic Measurements for Process Control", loc. cit.). However, in this case the same disadvantages resulting from Snell's law appear as in the case of ultrasonic flowmeters in which the ultrasonic transducers are attached to the measuring tube from the outside, therefore in the case of the so-called "clamp-on arrangement". In addition, there are problems with the acoustic impedance and technological problems with the plastic filling the transducer pockets, particularly at higher temperatures. The disadvantages and problems connected with filling the transducer pockets with plastic are the reason why this design has not been used in practice.